Scan for all matches in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using scan?

Consider the following Ruby code that uses scan to find all matches of digits in a string. What will be printed?

Ruby

text = "Order 123, item 456, code 789"
matches = text.scan(/\d+/)
puts matches.inspect

A ["1", "2", "3", "4", "5", "6", "7", "8", "9"]

B [123, 456, 789]

C ["123", "456", "789"]

D nil

💡 Hint

Remember that scan returns an array of strings matching the pattern.

The scan method finds all substrings matching the regex /\d+/, which means one or more digits. It returns them as strings in an array.

❓ Predict Output

intermediate

What does this scan with capture groups return?

Given this Ruby code, what is the output of p result?

Ruby

text = "Name: John, Age: 30; Name: Jane, Age: 25"
result = text.scan(/Name: (\w+), Age: (\d+)/)
p result

A [["John", "30"], ["Jane", "25"]]

B ["John", "30", "Jane", "25"]

C ["Name: John", "Age: 30", "Name: Jane", "Age: 25"]

D []

💡 Hint

When scan uses capture groups, it returns an array of arrays with the captured parts.

The regex has two capture groups: one for the name and one for the age. scan returns an array of arrays, each inner array holding the captures for one match.

🔧 Debug

advanced

Why does this scan code raise an error?

Look at this Ruby code snippet. It raises an error. What is the cause?

Ruby

text = "abc123def456"
matches = text.scan(/(\d+)/).map(&:to_i)
puts matches

A NoMethodError because <code>to_i</code> cannot be called on an array

B NoMethodError because <code>map</code> is called on a string

C No error; it prints [123, 456]

D No error; it prints [["123"], ["456"]]

💡 Hint

Check what scan returns when there are capture groups and how map(&:to_i) works on the result.

scan(/(\d+)/) returns an array of arrays: [["123"], ["456"]]. map(&:to_i) calls to_i on each inner array, but Array has no to_i method, raising NoMethodError.

📝 Syntax

advanced

Which option correctly uses scan to find all words starting with 'a'?

Which Ruby code correctly finds all words starting with the letter 'a' in the string "apple banana apricot cherry"?

Ruby

text = "apple banana apricot cherry"

A text.scan(/\b[a-z]+/)

B text.scan(/a\w*/)

C text.scan(/\bA\w*/i)

D text.scan(/\ba\w*/)

💡 Hint

Use word boundary \b and match words starting with 'a'. Case sensitivity matters.

Option D matches words starting with 'a' using \b for word boundary and a\w* for words starting with 'a'. Option D misses the word boundary, so it matches 'a' anywhere. Option D matches any word starting with any letter. Option D matches words starting with 'A' or 'a' but uses case-insensitive flag which is not required here.

🚀 Application

expert

Extract all email usernames from a string using scan

You have a string containing emails: "[e-mail], [e-mail], [e-mail]". Which Ruby code extracts only the usernames (the part before '@') using scan?

Ruby

emails = "[e-mail], [e-mail], [e-mail]"

A emails.scan(/\w+@\w+\.\w+/)

B emails.scan(/(\w+)@\w+\.\w+/).flatten

C emails.scan(/(\w+@)/).map { |m| m[0].chomp('@') }

D emails.scan(/(\w+)@/)

💡 Hint

Use capture groups to get the username part before '@'. Flatten the result if needed.

Option B uses a capture group for the username and matches the full email pattern. scan returns an array of arrays, so flatten is needed to get a simple array of usernames. Option B returns full emails. Option B tries to capture username with '@' included and then removes '@' manually. Option B returns array of arrays but without flattening, so the result is nested.

Practice

1. What does the Ruby method scan do when used on a string?

easy

A. It counts the number of characters in the string.

B. It replaces all matches of a pattern with another string.

C. It finds all matches of a pattern and returns them in an array.

D. It converts the string to uppercase.

Solution

1. Step 1: Understand the purpose of scan

   The scan method searches the string for all occurrences of a pattern.

2. Step 2: Identify the return value of scan

   It returns an array containing all matched parts of the string.

3. Final Answer:

   It finds all matches of a pattern and returns them in an array. -> Option C

4. Quick Check:

   scan returns matches array [OK]

Hint: Remember: scan returns all matches as an array [OK]

Common Mistakes:

• Confusing scan with gsub (replacement)
• Thinking scan returns a count
• Assuming scan modifies the original string

2. Which of the following is the correct syntax to find all digits in a string str using scan?

easy

A. str.scan(/d+)

B. str.scan("\\d+")

C. str.scan('\d+')

D. str.scan(/\d+/)

Solution

1. Step 1: Recognize the correct regex syntax in Ruby

   Ruby uses forward slashes /pattern/ to denote regular expressions.

2. Step 2: Identify the correct regex for digits

   \d+ matches one or more digits. The pattern must be inside slashes for regex.

3. Final Answer:

   str.scan(/\d+/) -> Option D

4. Quick Check:

   Regex in slashes with \d+ = str.scan(/\d+/) [OK]

Hint: Use /pattern/ for regex, not strings, in scan [OK]

Common Mistakes:

• Using string quotes instead of regex slashes
• Missing escape backslash in regex
• Using incorrect regex delimiters

3. What is the output of the following Ruby code?

text = "cat bat rat"
matches = text.scan(/.at/)
puts matches.inspect

medium

A. ["at", "at", "at"]

B. ["cat", "bat", "rat"]

C. ["c", "b", "r"]

D. []

Solution

1. Step 1: Understand the regex /.at/

   The dot . matches any single character, so .at matches any three-letter word ending with 'at'.

2. Step 2: Apply scan on the string

   The string "cat bat rat" contains "cat", "bat", and "rat" which all match the pattern.

3. Final Answer:

   ["cat", "bat", "rat"] -> Option B

4. Quick Check:

   scan finds all 3-letter words ending with 'at' [OK]

Hint: Dot matches any char; scan returns all full matches [OK]

Common Mistakes:

• Thinking dot matches zero or multiple chars
• Expecting partial matches like 'at' only
• Confusing scan output with string split

4. The following code is intended to find all words starting with 'a' in sentence. What is the error?

sentence = "apple banana apricot avocado"
words = sentence.scan("a\w+")
puts words.inspect

medium

A. The pattern should be a regex, not a string.

B. The pattern should use double backslashes.

C. The method scan does not work on strings.

D. The variable name 'words' is invalid.

Solution

1. Step 1: Check the argument type for scan

   The scan method expects a regex pattern, not a plain string.

2. Step 2: Identify correct regex usage

   The pattern should be written as /a\w+/ to match words starting with 'a'.

3. Final Answer:

   The pattern should be a regex, not a string. -> Option A

4. Quick Check:

   scan requires regex pattern, not string [OK]

Hint: Use /pattern/ for regex in scan, not string quotes [OK]

Common Mistakes:

• Passing string instead of regex to scan
• Misunderstanding backslash escaping
• Assuming scan modifies the string

5. Given the string data = "log12 ID123, ID456, ID789", how can you use scan to extract only the numeric parts after each 'ID' (e.g., ["123", "456", "789"])?

hard

A. data.scan(/ID(\d+)/).flatten

B. data.scan(/\d+/)

C. data.scan(/ID\d+/)

D. data.scan(/(ID\d+)/)

Solution

1. Step 1: Understand the pattern to extract numbers after 'ID'

   The regex /ID(\d+)/ matches 'ID' followed by digits, capturing only digits in parentheses. This ignores other digits like '12' in 'log12'.

2. Step 2: Use scan and flatten the result

   scan returns an array of arrays for capture groups, so flatten converts it to a simple array of strings.

3. Final Answer:

   data.scan(/ID(\d+)/).flatten -> Option A

4. Quick Check:

   Capture group extracts digits, flatten simplifies array [OK]

Hint: Use capture groups and flatten to get inner matches [OK]

Common Mistakes:

• Using data.scan(/\d+/) which grabs all digits including extras like '12' and '78'
• Using scan without flatten, getting nested arrays
• Matching full 'ID123' instead of just digits